High precision automated manufacturing, such as computer aided manufacturing (CAM), requires that the position of a tool be precisely known in space. However, most machine tools today fail to provide the necessary precision, in part due to their limited range of movement, high cost, inability to recognize their own work space to plan complex motion activity, and unrealistically high expectations of technical expertise available for the users of CAM software. To overcome these problems, engineers have developed mechanical movement systems that include a movable platform that is mounted with substantial freedom in translation and rotation by means of a plurality of legs the length and angular orientation of which are controllably adjustable. Through coordinated control of the effective length of the legs, the platform can be precisely moved through space. A tool can be mounted on to the platform and carried thereby to a precisely selected position and orientation. One such system is described in U.S. Pat. No. 5,575,597 entitled Mechanical Manipulator, the teachings of which are herein incorporated. As shown therein, one such movable platform system has three pairs of supportive legs coupled to a platform at triangularly spaced-apart locations by means of three universal joints each of which couples to one end of each of the two legs of the respective pair, and the two legs of each pair extend from their respective universal joint in divergent directions to spaced-apart locations in a mounting where each leg is drivingly engaged by a motor for moving the leg in its own longitudinal direction, thereby changing the effective length of the leg. Six such motors are mounted at spaced-apart locations in the mounting in a manner such as to accommodate the pivotal movements of the legs that will accompany their effective length variations.
Each pair of legs thus defines a triangle of support for the platform with the apex of the triangle coupled to the platform by means of the universal joint, the base of the triangle defined by the separation between the mounted motors which drive the respective legs, and the effective lengths of the legs being independently adjustable by operation of the motors. With the platform supported by three such support triangles and the effective lengths of all six legs independently adjustable, the position and orientation of the platform becomes infinitely adjustable within the limits of accommodation of the universal joints and the motor-mountings. Thus the tool mounted to the platform can be positioned with high precision and moved with a great range of freedom.
Although these movable structures can work well, the supportive leg assemblies are generally quite complex, costly and difficult to manufacture. Moreover, the complexity of the legs, typically including inter-locking members, are subject to problematic thermal expansion, that can change the length of the extension arm and therefore interfere with the accurate placement of the point on the platform. In addition, these structures have limits within which the platform may be positioned and oriented. To achieve rotational orientation about an axis perpendicular to their platforms, these systems generally require that the platform carry an additional rotational stage with an additional motor or other actuator.